1. Field of the Invention
The present invention relates to a type of steam distributor known as a "steambox" which is used in the manufacture of sheet materials, and in particular, to a steambox design which increases the production rate and quality of manufactured paper sheet.
2. Description of Related Art
One of the parameters used in grading sheet materials is the moisture content of the material For example, in the paper production process, various grades of paper having different moisture contents are produced to suit various applications.
Paper production typically begins with a wet mass of wood pulp fibers, called a "slurry". The slurry is laid out in a flat sheet. Water is then removed from the slurry, in stages, using several different techniques until a dry usable sheet of paper is obtained. FIG. 1 illustrates an example of such a papermaking process. In particular, FIG. 1 illustrates, in highly simplified cross-sectional form, a Fourdrinier-type papermaking machine 10. The papermaking machine 10 includes a headbox 12 which discharges the slurry 14 to a porous conveyer belt 16 called a "wire". In the first stage of drying, water is drained from the pulp through the wire 16. The paper sheet 18 then travels between a porous vacuum roll 20 and the faceplate 22 of a steambox 24. This steambox 24 jets steam against the sheet 18. As shown by arrows 25, the steam is drawn from the steambox 24, through the sheet 18 and into the vacuum roll 20. Water from the slurry is also drawn into the vacuum roll 20.
The steam treatment may cause an increase in sheet temperature of approximately 30.degree. C. The increased temperature decreases the viscosity of the water in the sheet 18 so that the water can be efficiently squeezed and/or suctioned out of the paper sheet 18 in a subsequent section of the papermaking machinery known as the "press section" 28. The design of the press section 28 can vary greatly between different papermaking machines Typically, however the press section 28 will include a number of rolls 30 which squeeze water out of the sheet 18 and endless felt belts 32 which absorb water from the sheet 18. Because the steam heats the water, and heated water has decreased viscosity, the pressing and sectioning of water out of the sheet is rendered more effective
In still later drying steps, the sheet 18 is typically passed over several heated drums 34 in the so-called "dryer section" 36 of the papermaking machine 10. These drying drums 34 are typically hollow and made of steel. The drums 34 are heated by steam flowing through the drums 34.
In the production of many paper products, it is desirable to automatically control the "cross-directional" (i.e., the direction across the width of the sheet perpendicular to the direction of sheet movement) moisture content of the paper sheet using a steambox. Many papermaking machines have scanning moisture sensors which continuously scan back and forth across the width of the sheet and sense the sheet moisture content at various locations across the sheet as the paper is being manufactured. Such a moisture sensor 38 is schematically illustrated in FIG. 1. The moisture sensor 38 may be mounted to a carriage (not shown) which travels back and forth across the width of the sheet 18. The moisture content of the sheet is thus measured at intervals or "slices" across its width. The information from this continuous moisture measurement can be fed to a computerized controller 40, such as the main process control computer for the papermill. The controller then separately and individually controls the amount of steam applied by the steambox to various slices of the sheet based upon the sensed moisture content.
Examples of automatically controllable steam distributors are shown in U.S. Pat. Nos. 4,253,247 and 4,580,355, which are incorporated herein by reference These patents teach multi-chambered steam distributors in which steam flows from a steam pipe through a valve associated with each chamber, into each chamber, and then is directed to the sheet slice adjacent each chamber. The steam flow out of each chamber and toward an adjacent slice of the sheet is controlled by progressively opening or closing the associated valve.
A typical steambox is divided into sections, such that each section spans only a few inches across the width of the sheet. Each section of the steambox is separately controllable to jet varying amounts of steam against the opposing portion of the sheet Based upon the measured moisture profile of the sheet across its width, the controller adjusts the amount of steam jetted from the steambox at each cross-directional section or slice to achieve the desired moisture profile along the cross-direction of the sheet. As more steam is jetted from a particular chamber of the steambox onto an opposing slice of the sheet, that slice becomes hotter and the water viscosity in that slice therefore decreases; thus, the more effectively water may be removed from the hot sheet. In other words, the use of more steam results in a drier sheet section Conversely, as less steam is applied to a slice, the drying action of the vacuum roll 20 and press section 28 becomes less efficient, and thus that slice retains more moisture.
According to the design of certain steamboxes, the faceplate of the steambox is perforated with numerous small steam holes to provide an even distribution of the steam across the width of any one slice. For example, a typical faceplate may include four 1/8 inch diameter holes per square inch. Unfortunately, however, since papermaking is a high speed process, dust, composed mostly of paper fibers and other material used to make paper, such as mineral fillers, tends to fly off the surface of the sheet during production. This dust tends to accumulate on all available surfaces of the papermaking machinery, including the faceplate of the steambox. The accumulation of such dust is especially prevalent in the manufacture of relatively light paper grades having a porous texture, such as tissue paper. In any event, the dust clogs the steam holes in the steambox faceplate
This clogging of the steam holes can be extremely deleterious to the papermaking process. First, as the holes become clogged, less steam escapes from the steambox to impinge upon and heat the sheet. As a result, heating efficiency and subsequent sheet drying efficiency are decreased. Therefore, the paper production rate must be slowed since the paper will need to pass more slowly through the dryer section. Second, should the dust tend to accumulate preferentially on certain sections of the faceplate, steam may be prevented from leaving only those sections of the faceplate Thus, control of the cross-directional moisture content of the sheet is adversely affected.
Because of the accumulation of dust on the papermaking machinery and its adverse affect on the papermaking process, the papermaking machine must be cleaned periodically. Conventionally, the papermaking machinery is shut down and cleaned manually. However, this is a time consuming and therefore costly process. This is particularly true for modern high speed and expensive papermaking machines, where it is frequently essential for profitability that the machines be operated almost continuously. Accordingly, it would be desirable to automate the cleaning process and thus minimize "down-time" spent cleaning the machinery.